Method and arrangement for the adaptive filtering of signals

ABSTRACT

The invention relates to a method for adaptive filtering in which a level of a reference signal necessary for the adaptive filtering is determined in the context of an adaptation by estimation, using a quantity equivalent to the reference signal, wherein the filtering is dependent on the electrical power of at least one signal extracted from a transmission channel to be filtered, and the equivalent quantity is determined by means of a mathematical function based on the electrical power of at least the extracted signal. The invention further relates to an arrangement with means for carrying out the method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the United States national phase under 35 U.S.C. §371 of International Patent Application No. PCT/DE2006/002006, filed onNov. 15, 2006. That application is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for the adaptive filtering of signalsas well as an arrangement for the adaptive filtering of signals.

2. Background of the Art

A known method for performing adaptive filtering consists of using anoutput signal of a filter, used especially as a reference, as areference signal for comparison with an output signal of a filtercarrying out the adaptive filtering, and adjusting the parameters of theadaptive filter on the basis of the comparison results.

With known adaptive filters different approaches are followed. Forexample, some adaptive filters provide a compromise, one between arequirement at a fixed adaptation speed to adapt as quickly as possible,if only the echo is present, and a requirement not to regulate theadaptive filter during cross-talk, with this occurring more frequentlythe greater the adaptation speed.

There is a problem particularly during the use of adaptive filters withso-called echo cancellers, which are used primarily intelecommunications and in particular during voice transmission—theso-called Voice over IP (VoIP)—that the reference signal is superimposedon greatly varying interference signals. In this field of applicationone speaks of so-called double-talk (cross-talk), the simultaneousspeaking of two remote participants in the communication. Here thereference signal represents the superimposition of local signals, theso-called near-end signals of the local participant, and the echo fromit on the part of the remote participant.

An optimal adaptation behavior therefore requires that the speed of theadaptation be adapted to the relationship of reference signal to theinterference signal, with the level of the reference signal usuallyneeding to be determined.

For example, a method known from EP 1 320 941 B1 consists of estimatingan amplification factor between a residual echo occurring on thetransmission channel, used as the reference signal, and a remote,so-called far-end signal and determining the level of the residual echofrom it. Other methods again rely on an estimate of the residual echobased on the correlation between the near-end signals and the far-endsignals.

From U.S. Pat. No. 6,226,380 B1 is known a method and arrangement ofdistinguishing between echo path change and double talk conditions in anecho canceller.

BRIEF SUMMARY OF THE INVENTION

The task of the present invention is to provide an optimized method forthe echo cancellation as well as an arrangement for the adaptivefiltering.

According to the inventive method, a level of a reference signalnecessary for the adaptive filtering is determined in the context of theadaptation, using a quantity equivalent to the reference signal, withthe filtering being dependent on the electrical power of at least onesignal extracted from the transmission channel to be filtered and theequivalent quantity of the extracted signal being determined by means ofa mathematical function based on the electrical power of at least theextracted signal. Understood by power is the so-called quadratic meanvalue in digital signal processing, because this represents the basis ofa power calculation.

Preferably the mathematical function uses advantageously an initialpower from a signal (S_(in)), emitted by a local participant, and anecho signal from the transmission channel, producing the firstelectrical power, to determine the equivalent quantity, since thesesignals already represent good indicators.

Alternatively, or advantageously in addition, a further embodiment forthe determination of the equivalent quantity is given so that themathematical function uses a second electrical power resulting from adifference signal generated from a signal emitted by the localparticipant and an echo signal from the transmission channel and acompensated echosignal.

Alternatively and preferably in addition to one of the aforementionedembodiments, a further advantageous optimization may be obtained, if themathematical function uses an third electrical power of the echo signalfrom the transmission channel, estimated by the adaptive filter, for thedetermination of the equivalent quantity.

As a rule, the embodiment, where the determination of the equivalentquantity by the mathematical function is done as an estimate of a fourthelectrical power of the echo signal from the transmission channel, thisestimate being based on the first electrical power, the secondelectrical power and the third electrical power, is an advantage, sincean estimate based on these three quantities enhances the effectivenessof the adaptation, particularly during the use of the inventive methodfor echo cancellation.

Preferably the estimate is done according to the formula

$P_{{Echo},{est}} = {P_{diff} \cdot \sqrt{\frac{P_{Sin}}{P_{est}}}}$with $P_{diff} = {\frac{1}{2} \cdot \left( {P_{Sin} - P_{err}} \right)}$

defined, with

P_(sin) the first electrical power,

P_(err) the second electrical power,

P_(est) the third electrical power, and

P_(Echo,est) the fourth electrical power

An estimate according to the abovementioned formula has excellentproperties re: the speed and effectiveness of the adaptive filtering, inparticular when the inventive method is used for echo cancellationtasks, since, as in simulations, they enable an outstandingly preciseestimate of the reference signal.

If the adaptive filter is operated as an echo canceller, and if theadaptive filter based on a balancing of a fifth electrical power atwhich the echo elimination signal, used as a summing point to generatethe output signal of the adaptive filter, is reset into an initial stateof the adaptation, echo changes, which otherwise are difficult todistinguish from the double-talk interference caused by the simultaneousspeaking of participants, are detected significantly better. Due to thedetection and resetting, a new adaptation of the filter is started thenin sufficient time, if a detuning of the adaptation process, i.e., ageneration of erroneous coefficients, threatens.

Preferably a resetting occurs by the absolute quantity of a fifthelectrical power exceeding a first threshold value so that there areadditional degrees of freedom in the optimization of the method forsetting a particularly suitable threshold value, with this being donethrough simulations or through experimental trial runs.

In a further advantageous embodiment of the method, the resetting occurswhen the duration during which the threshold value is exceeded withoutinterruption attains a second threshold value defining a period. As aresult only briefly occurring peaks are captured and prevented thattrigger a new adaptation without justification. In addition, this alsooffers an opportunity for the method to improve the detection ratethrough a skilled selection or setting of the second threshold value,which again can be done through simulations or through experimentaltrial runs.

In a further embodiment the first threshold value is defined based on aproportional value indicative of double-talk as well as based on thesecond electrical power and third electrical power. As a result, theadvantageous degree of freedom of the first threshold value is exploitedoptimally so that, based on the actually occurring quantities, asmentioned, automatically adjusted and adapted in the embodiment,erroneous detections are clearly minimized or almost eliminated. Thus anembodiment in which the determination of the first threshold value isdefined according to the formula

P _(Δ) =s·√{square root over ((P _(est) ·P _(err)))}

with

s the proportional value for the indication,

Perr the second electrical power,

Pest the third electrical power and

P_(Δ) the fourth electrical power

turns out to be particularly advantageous.

The inventive adaptive filter supports in an advantageous manner theexecution of the inventive method through means implementing thepreviously described approaches.

Additional details as well as advantages of the invention are explainedin greater detail based on the embodiments represented in FIGS. 1 and 2.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of an echo canceller implementing theinventive method, used to optimize electrical powers determined for thereference signal estimation,

FIG. 2 shows a block diagram of an echo canceller implementing theinventive method, used to optimize electrical powers determined for thepower balance based detection of echo changes and the control of thecanceller.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 the block diagram of an echo canceller implementing theinventive method is represented, which is used to optimize theelectrical powers determined for the reference signal estimation.

Thus in the schematic representation an echo canceller according to theinvention is depicted between the echo path, i.e., the arrow between afirst Port S_(Out) at which a voice signal to be sent on the echo pathis emitted and a second Port S_(in) at which an echo signal arrives fromthe echo path as well as a third Port R_(in), where the voice signals tobe received from the echo path are inputted and then emitted to the echopath via a fourth Port R_(Out).

Depicted in addition is an adaptation unit AB for doing the adaptivefiltering, which generates an estimated echo signal, necessary for thecoefficients for the adaptation and an output signal based thereupon,which is conducted to a summer SUM for the purpose of eliminating anecho, where it is deducted from an echo affected signal received on theecho path via the second Port S_(Out) and, completely cleaned of theecho in the ideal case, is emitted via the first Port S_(Out).

Furthermore, it should be recognized that the adaptation unit, hasprocessing units, known from prior art, that are required for this, withat least the adaptation control AC being modified in order to implementthe inventive method. To control the adaptation unit AS, as may be seenfrom the representation, the electrical power of an echo signalP_(Echo,est), estimated by means of the inventive method, as well as theelectrical power of a near-end signal P_(sin), are fed, along with theusual additional signals, to the adaptation control AC.

Because of these signals the adaptation control AC is able to calculateoptimally adjusted coefficients, which makes it possible to improve theecho cancellation.

According to the invention, the electrical power levels P_(sin),P_(err), P_(est) of the signals mentioned are determined by means of apower determining device EE from an output signal, comprised of aresidual echo and a signal from the near-end participant, a near-endsignal, comprised of an echo and the signal from the near-endparticipant, and from the estimated echo signal. Then they aretransmitted to an estimation device ES designed according to theinvention.

Based on the formula,

$P_{{Echo},{est}} = {P_{diff} \cdot \sqrt{\frac{P_{Sin}}{P_{est}}}}$with $P_{diff} = {\frac{1}{2} \cdot \left( {P_{Sin} - P_{err}} \right)}$

the echo estimation device ES estimates the electrical power of the echowhich, as explained above, serves as a basis for regulating theadaptation control AS.

FIG. 2 also shows a block diagram of an echo canceller, in which theidentical elements are labeled the same in keeping with the previousfigure description,

In contrast to what was described before, the echo cancellerimplementing the inventive method shown in the representation uses thedetermined electrical power to optimize the power balance baseddetection of echo changes and the control of the canceller.

The embodiment of the invention represented also has a power determiningdevice, which generates the same electrical power levels P_(sin),P_(err), P_(est) from the abovementioned signals.

Unlike the first embodiment, these are passed along alternatively orsupplementarily to a feed to an echo estimator on a detection device forthe detection of echo changes, which the electrical power levelsP_(sin), P_(err), P_(est) balance according to

P _(Δ) =P _(sin) +P _(err) −P _(est)

with

P _(Δ) =s·√{square root over ((P _(est) ·P _(err)))}

with s being the result of a rough detection of a cross-talk effect,also called “double talk.” A balancing at the summing point SUM of theecho canceller is carried out, which is fully in keeping with therequirements that the G168-2004 has set for a line echo canceller, sinceit makes a detection of echo changes and a requisite new adaptationpossible within a few seconds as needed. In addition, this method ischaracterized by the fact that very little computing power is necessary,and thus optimal function is also ensured for applications that have alarge number of channels.

1. A method for adaptive filtering, in which a level of a referencesignal necessary for the adaptive filtering is determined in the contextof the adaptation by an estimation using a quantity equivalent to thereference signal(P_(Δ), P_(Echo,est)), comprising: a) filtering asignal, wherein the filtering is dependent on the electrical power(P_(sin), P_(err), P_(est)) of at least one signal (S_(in), S_(out),R_(in), R_(out)) extracted from the transmission channel to be filtered,b) determining the equivalent quantity (P_(Δ), P_(Echo,est)) by amathematical function based on the electrical power (P_(sin), P_(err),P_(est)) of at least the extracted signal(S_(in), S_(out), R_(in),R_(out)); and c) estimating a fourth electrical power (P_(Echo,est)) ofthe echo signal of the transmission channel is done by the mathematicalfunction, with this being estimated based on the first electrical power(P_(sin)), the second electrical power (P_(err)) and a third electricalpower (P_(est)).
 2. The method of claim 1, wherein to determine theequivalent quantity (P_(Δ), P_(Echo,est)), the mathematical functionuses a signal (S_(in)) emitted by a local participant and an echo signalfrom the transmission channel producing a first electrical power(P_(sin)).
 3. The method of claim 1, wherein to determine the equivalentquantity (P_(Δ), P_(Echo,est)) the mathematical function uses adifference signal from a signal (S_(in)) emitted by a local participantand an echo signal from the transmission channel plus a compensated echosignal producing a second electrical power (P_(err)).
 4. The method ofclaim 1, wherein to determine the equivalent quantity (P_(Δ),P_(Echo,est)), the mathematical function uses a third electrical power(P_(est)) estimated by the adaptive filter.
 5. The method of claim 1,wherein the estimate is defined according to the formula:$P_{{Echo},{est}} = {P_{diff} \cdot \sqrt{\frac{P_{Sin}}{P_{est}}}}$with $P_{diff} = {\frac{1}{2} \cdot \left( {P_{Sin} - P_{err}} \right)}$with P_(sin) the first electrical power, P_(err) the second electricalpower, P_(est) the third electrical power, and P_(Echo,est) the fourthelectrical power.
 6. The method of claim 1, further comprising a)operating the adaptive filter as an echo canceller, b) resetting theadaptive filter into an initial state of the adaption, based on abalancing of a fifth electrical power (P_(Δ)), at which the echoelimination signal, used as a summing point to generate the outputsignal (S_(out)) of the adaptive filter.
 7. The method of claim 6,wherein the resetting occurs by exceeding a first threshold value(P_(Δ, max)) by the absolute quantity of a fifth electrical power(P_(Δ)).
 8. The method of claim 7, wherein the resetting occurs when theduration during which the threshold value is exceeded withoutinterruption attains a second threshold value defining a period.
 9. Themethod of claim 7, comprising determining the first threshold value(P_(Δ, max)) based on a proportional value (s) indicative of double-talkas well as on the second electrical power (P_(err)) and the thirdelectrical power (P_(est)).
 10. The method of claim 9, wherein thedetermination of the first threshold value is defined according to theformulaP _(Δ) =s·√{square root over ((P _(est) ·P _(err)))} s the proportionalvalue for the indication, Perr the second electrical power, Pest thethird electrical power, and P_(Echo,est) the fourth electrical power.11. An adaptive filter comprising a filter and a means to implement themethod of claim 1.